A base station and a mobile terminal that perform radio communication using a time division duplex (TDD) method transmit signals in upstream and downstream link directions in a time dividing manner (a same frequency range can be used for transmission and reception). For example, the base station and the mobile terminal perform full-duplex communication by alternately switching between a transmit period of a downstream link direction and a transmit period of an upstream direction within one radio frame. A transmission frame in a time division duplex method includes, for example, a downstream link subframe (DL subframe) with which a signal of a downstream link is transmitted, and an upstream link subframe (UL subframe) with which a signal of an upstream link is transmitted, as illustrated by FIG. 1. Communication systems with these requirements include WiMAX (Worldwide Interoperability for Microwave Access) (see, for example, IEEE 802.16e-2005).
In a time division duplex method, a frame start timing is detected for synchronizing a timing of switching between a transmit period of a downstream link direction and a transmit period of an upstream link direction between a base station and a mobile terminal.
Also, in an orthogonal frequency division multiplexing (OFDM) modulation method, OFDM symbols generated by modulating a plurality of subcarriers are transmitted. In order to demodulate each of the OFDM symbols, a base station and a mobile terminal perform a symbol synchronizing process to detect a symbol start timing, as illustrated by FIG. 2.
At the head of each frame transmitted in time division duplex OFDM modulation communication, a known certain pattern called a “preamble” is included. A receiving device applicable to a time division duplex OFDM modulation communication system will now be described with reference to FIG. 3.
At a symbol start timing detecting portion 12, correlation calculation in a time domain is performed between a cyclic prefix (CP) in an OFDM symbol and a rear portion of an effective symbol from which the CP is copied to detect a start timing of an OFDM symbol. Then, at a frame start timing detecting portion 14, correlation calculation in a frequency domain is performed between each symbol and a preamble pattern to detect a preamble symbol. The frame start timing detecting portion 14 outputs a detection start timing of the preamble symbol as a frame start timing.
In the above-described receiving device, correlation calculation in a frequency domain is performed per symbol. Thus, the amount of correlation calculation is very large.
As an example, a case of a WiMAX receiving device operating at 10 MHz frequency range will now be described with reference to FIG. 4. FIG. 4 illustrates an example of a frame format applicable to WiMAX. As illustrated in FIG. 4, one frame contains a DL subframe and a UL subframe. One frame period is 5 ms, for example. Also, a DL subframe contains 35 OFDM symbols and a UL subframe contains 12 OFDM symbols, for example. One OFDM symbol period is 1,152 Ts, a gap period of switching from transmitting to receiving is 1,104 Ts, and a gap period of switching from receiving to transmitting is 672 Ts, where “Ts” indicates a sample length. For example, a sample length is essentially equal to the reciprocal of a sampling frequency (Fs), which is about 11.2 MHz, and is about 89 ns. The gap of switching from transmitting to receiving may be called TTG (Transmit/Receive Transition Gap), and the gap of switching from receiving to transmitting may be called RTG (Receive/Transmit Transition Gap).
In the frame format illustrated in FIG. 4, the number of OFDM symbols per frame is 47, including OFDM symbols contained in a DL subframe and OFDM symbols contained in a UL subframe. Also, the number of patterns of preambles is 114.
Thus, in this receiving device, correlation calculations are conducted 47×114=5,358 times.